1. Field of the Invention
This invention generally relates to pilot tone acquisition and timing offset estimation in a wireless communication system such as that used for cellular telephony. It provides a method and apparatus for detecting a synchronization signal from the radio propagation channels with sufficiently high probability as well as an acceptably low false alarm rate even in the presence of a large frequency deviation at the receiving side. In addition, aspects of the present invention locate the timing boundary of the incoming data stream in a digital transmission system by detecting the pilot tone.
2. Description of the Related Art
In a mobile communication system having a pilot channel, a mobile station (MS) synchronizes with the base station (BS) via a pilot channel so that information can be exchanged and a communication link can be established successfully. Since the mobile station generally does not have a sufficiently accurate timing and frequency reference, the mobile station detects the synchronization signal in the pilot channel and extracts from the pilot signal a frequency offset for tuning the mobile station's internal frequency reference and tuning the timing offset for frame synchronization. More particularly, in a time division multiple access (TDMA) system like the European Global System for Mobile Communication (GSM), a logical channel called the frequency correction channel (FCCH) is transmitted on the broadcast control channel (BCCH) carrier and interleaved in time within the structure of the fifty-one frame multiframe to facilitate coarse frequency and timing synchronization. When the mobile station is powered on or when the mobile station is handed off from one cell base station to another, the mobile station has to capture the FCCH from the interested base station to communicate with that base station.
The FCCH provides mobile stations with the ability to initially synchronize with a system employing the GSM standard. An appropriate physical manifestation of the logical FCCH is generated by providing a constant input sequence into a Gaussian minimum shift keying (GMSK) modulator in a repeated manner, as is described in detail in GSM Specification 05.02, version 8.3.0 (1999). The physical manifestation of the FCCH in a GSM system includes the frequency correction burst (FCB). The modulated FCCH waveform is a pure tone of about 66.7 kHz. By detecting the existence and timing of this pure tone, the mobile station can identify the frame boundary and then synchronize its local oscillator with respect to the interested base station. After detecting the FCCH, the mobile station uses the information obtained from detecting the FCCH to decode the synchronization burst (SB) located eight bursts after the FCCH and finally the mobile station can lock onto (i.e., camp on) the interested base station using the system information on the broadcast control channel.
Since the duration of the FCCH is relatively short, FCCH detection needs to occur in real time even for bad channel conditions such as severe noise perturbations and large frequency deviations. U.S. Pat. No. 6,356,608 proposes a method for detecting and synchronizing with the FCCH in a receiver's back-end circuitry. That patent's strategy relies on pre-processing to provide signal de-rotation via table lookup and uses a large memory to perform the de-rotation, which can be undesirable if it impacts on the speed with which the synchronization tone is acquired. The apparatus in U.S. Pat. No. 5,241,688 adaptively filters received signals to acquire the timing and frequency offset compensation. Some potential problems have to be taken into account in using an adaptive filtering strategy. The transient processing of the training phase of the adaptive filtering may introduce a delay that could affect the FCCH detection algorithm. Furthermore the filter parameters may go out of bounds after several TDMA frames of unsuccessful detection so that the FCCH might not be detected.